Process for filtering an electrical signal by charge transfer into a semiconductor and switch capacitor filter using such a process

ABSTRACT

The process more particularly comprises introducing beneath the final storage capacitor C 6  a quantity of charges equal to that located there during the processing of the preceding sample of the input signal, prior to the beginning of charge transfer from said capacitors to the means which convert the charges into a voltage, and distributing the charges between said capacitor and the preceding capacitor as a function of the capacitance values. The switched capacitor filter more particularly comprises, when it is looped, a circuit R permitting the application to the looping capacitor C 4  of a voltage V g  +ΔV s , while the foils of the other storage capacitors of the filter are connected to the voltage V g .

BACKGROUND OF THE INVENTION

The present invention relates to a process for filtering an electrical signal by the transfer of charges into a semiconductor. It also relates to switched capacitor filters using such a process.

Active filters called switched capacitor filters are known, particularly from two articles in the American "IEEE Journal of solid-state circuits", vol. SC-12, No. 6, December 1977, pp. 592 to 608.

Switched capacitor filters generally have at least one amplifier associated with a network of MOS switches and capacitors, in which the combination of two MOS switches and one capacitor serves as the resistor. Switched capacitor filters have small overall dimensions, as well as high linearity and temperature stability.

The capacitors of switched capacitor filters are integrated in per se known manner by a procedure involving two metallisation levels. Each terminal of the integrated capacitors then has a stray capacitance of non-negligible value in relation to the substrate. The stray capacitances created by the capacitors of the network associated with the amplifier are particularly disadvantageous, because they lead to errors in the frequency response of the filter. It is then necessary to increase the values of the capacitors of the system in order to reduce the effect of stray capacitances, which increases the surface of the semiconductor substrate necessary for producing the filter, and therefore its cost and overall dimensions.

BRIEF SUMMARY OF THE INVENTION

The switched capacitor filters using the filtering process according to the invention have the advantage of not having stray capacitances on the capacitors of the network associated with the amplifier, because they are not produced by the method involving two metallisation levels and instead have a common terminal and connection between the said capacitors takes place by charge transfer. It is therefore possible to use low values for the capacitors of the system so that the cost and overall dimensions of the filter can be reduced.

The invention therefore relates to a process for filtering an electrical signal by transferring charges into a semiconductor substrate, which is covered with a first insulating layer on which is arranged a first group of electrodes constituting with the first insulating layer and the substrate charge storage capacitors, whilst a second group of electrodes is interposed with the first and is separated therefrom by a second insulating layer, the electrodes of said second group constituting with the first group, the second insulating layer and the substrate switches which control the transfer of charges between the storage capacitors, wherein the processing of each sample of the electrical input signal comprises the following stages:

(a) a quantity of charges corresponding to the sample of the electrical input signal during processing is introduced beneath the first capacitor in the charge transfer direction;

(b) beneath the last capacitor in the charge transfer direction is introduced a quantity of charges which is equal to that already there, during the processing of the preceding sample of the electrical input signal and prior to the beginning of the transfer of charges from said last capacitor to means which convert the charges into an electrical voltage;

(c) a common surface potential is established between two successive capacitors, whereby during the said stage the capacitors are brought to a constant potential V_(g) ;

(d) the charges are transferred from the last capacitor in the charge transfer direction to means which convert the charges into an electrical voltage.

The order of stages (a) and (b) in the process is random, but stages (a) and (b) must precede stage (c), which must precede stage (d).

In the case of filters of at least the second order, the filter is looped by one of the storage capacitors and the process comprises a supplementary stage (e). During this stage, a common surface potential is established between the looping capacitor and the following capacitor, in the charge transfer direction.

These capacitors are respectively brought to voltages V_(g) +ΔV_(s) and V_(g), in which ΔV_(s) is the variation in the output voltage of the filter for the preceding sample of the electrical input voltage. The order of stages (a), (b) and (e) is random, but these stages must precede stage (c), which must precede stage (d).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show:

FIG. 1 the diagram of a switched capacitor filter of the first order according to the prior art.

FIGS. 2a to 2f an embodiment of the filter of FIG. 1 in accordance with the present invention and diagrams illustrating the filtering process according to the invention.

FIGS. 3a to 3c diagrams of the signals liable to be applied to the filter of FIG. 2.

FIG. 4 the diagram of a switched capacitor filter of the second order according to the prior art.

FIG. 5 an embodiment of the filter of FIG. 4 according to the present invention.

FIGS. 6a to 6e diagrams of the signals likely to be applied to the filter of FIG. 5.

FIG. 7 another embodiment of the filter of FIG. 4 according to the invention and diagrams illustrating its operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, the same references designate the same components and for reasons of clarity, the dimensions and proportions of the various components have not been respected.

FIG. 1 shows the diagram of a switched capacitor filter of the first order according to the prior art. This filter is of the RC type and has between two points P₁ and P₂ a resistor by arranging in series two switches I₁ and I₂ and by a capacitor C₁ between the common point P₃ of I₁ and I₂ and earth. This filter also has a capacitor C₂ between point P₂ and earth and an amplifier of gain g between point P₂ and output S of said filter.

FIG. 2a shows an embodiment of the filter of FIG. 1 according to the present invention. FIG. 2a is a cross-sectional view work parallel to the charge transfer direction of the network of capacitors and MOS switches of the filter of FIG. 1, whilst the circuit diagram associated with said system is also shown. The filter according to the invention uses the transfer of charges into a semiconductor substrate 1, covered with a first insulating layer 11 on which is arranged a first group of electrodes, which create with the first insulating layer and the substrate storage capacitors for charges C. A second group of electrodes is interposed with the first group and from which it is separated by a second insulating layer 12. The electrodes of this second group constitute, with the first group, the second insulating layer and the substrate MOS switches I, which control the transfer of charges between storage capacitors.

The semiconductor substrate can be made from silicon, for example of type P and the two groups of electrodes can be of polycrystalline silicon. The second insulating layer 12 is then made by oxidizing the polycrystalline silicon of the first group of electrodes.

The filter shown in FIG. 2a has two storage capacitors C₁ and C₂, between which is inserted an MOS switch I₂. A grid G_(c), located in the same plane as capacitors C₁ and C₂, controls the current supplied by a diode D located in the semiconductor substrate and brought to a constant potential V_(D). The grid G_(C) is connected to a constant potential V_(DD), which exceeds V_(D). An MOS switch I₁ in the same plane as switch I₂ separates grid G_(C) from the first storage capacitors C₁ in the charge transfer direction indicated by an arrow.

Capacitor C₁ is connected to the common point of MOS transistors T₁ and T₂, which are arranged in series and respectively connected to the input signal E and the voltage V_(G). Capacitor C₂ is permanently connected to voltage V_(G), which is generally of the order of 5 V. The means which convert the charges into an electrical voltage comprise a charge reading capacitor C_(l), located in the same plane as capacitors C₁ and C₂ and separated from C₂.by MOS switch I₃ located in the same plane as switches I₁ and I₂. These means also comprise a circuit R connected at point P to capacitor C_(l) and which supplies the output voltage to point S. An MOS transistor T₃ is connected between point P and earth.

Following the reading capacitor C_(l) the surface potential of the substrate is fixed at zero by an MOS switch I₄, followed by a type P⁺ zone which is highly doped and of the same conductivity as the substrate located beneath the substrate surface. Switch I₄ and the P⁺ type zone are raised to the same reference potential as the substrate.

FIGS. 2b, c, d, e and f illustrate the filtering process according to the invention and will now enable us to describe the operation of the filter of FIG. 2a. These drawings show the state of the potential wells created in substrate 1 at various times t₁ to t₅ and for reasons of clarity only the interface 13 of the substrate with the first insulating layer 11 is shown.

It should also be noted that throughout the description the semiconductor substrate is of the P type and that the transferred charges are minority carriers (electrons). It is obvious that in the case of a type N substrate, in which the holes move, it is merely necessary to reverse the polarity of the potentials applied.

FIG. 3 shows diagrams of signals which can be applied to the filter of FIG. 2a. FIGS. 3a, b and c represent potentials φ_(A), φ_(B), φ_(L) applied to switch I₁ and transistor T₃ in the case of φ_(A), to switch I₂ and transistor T₂ in the case of φ_(B) and to switch I₃ and transistor T₁ in the case of φ_(L). The potentials φ_(A), φ_(B), φ_(L) are periodic functions of the same square-wave cycle T, whose amplitude varies between a low level and a high level. Potentials φ_(A) and φ_(B) are preferably identical, but phase-displaced in such a way as not to simultaneously be at the high level. Potential φ_(L) passes from the high level before φ_(A), with φ_(B) at the low level and passes to the low level at the same time as φ_(A).

FIG. 2b illustrates stage (a) as the process during which a quantity of charges corresponding to the sample of the electrical input signal E during processing is introduced beneath the first capacitor in accordance with the charge transfer direction C₁.

Switch I₁ and transistor T₁ are then conductive, whilst switch I₂ and transistor T₂ are blocked. A same surface potential equal to V_(D), the polarisation voltage of diode D, is established beneath the control grid G_(C) and beneath capacitor C₁.

FIG. 2b also illustrates stage (b) of the process during which a quantity of charges equal to that located there during the processing of the preceding sample of the electrical input signal and before the start of the transfer of charges from the last capacitor to the means converting the charges into an electrical voltage is introduced beneath the final capacitor in the charge transfer directions C₂.

Transistor T₃ is conductive and zeros the reading capacitor C_(l). Switch I₃ is also conductive and the quantity of charges stored beneath C_(l) is transferred beneath C₂, which is subject to potential V_(g).

The filter of FIG. 2a is in the state shown in FIG. 2b at time t₁ indicated at FIG. 3, such that φ_(A) and φ_(L) are simultaneously at the high level.

FIG. 2c shows the state of the filter of FIG. 2a at time t₂ when all the switches and transistors are blocked.

The surface potential beneath capacitor C₁ at time t₂ φ_(S1) (t₂) is substantially equal to its value V_(D) at time t₁.

In an MOS capacitor, the equation at a time t between the voltage V applied to said capacitor, the surface potential φ_(S) and the inversion charge Q beneath the capacitor is written: ##EQU1## with C being the capacitance value and ##EQU2## in which .sup.ε S is the dielectric constant of the semiconductor, q the electron charge and N_(A) the density of the acceptor ions in the semiconductor.

At time t₂, the inversion charges beneath the capacitors C₁ and C₂ are written: ##EQU3##

FIG. 2d illustrates stage (c) of the process during which a common surface potential is established between two successive capacitors, whereby during this stage the latter are raised to potential V_(g).

Switch I₂ and transistor T₂ are then conductive, whilst switches I₁ and I₃ and transistor T₁ are blocked.

The filter is in the state shown in FIG. 2d at time t₃, where only the potential φ_(B) is at the high level.

FIG. 2e represents the state of the filter at the time where all the potentials φ_(A), φ_(B), φ_(L) are at the low level.

Beneath capacitors C₁ and C₂ is established essentially the same surface potential φ_(S1) (t₄) and φ_(S2) (t₄), which is substantially equal to φ_(S) (t₃), the surface potential common to C₁ and C₂ at time t₃.

From t₂ to t₄, capacitors C₁ and C₂ are isolated from the remainder of the semiconductor substrate by switches I₁ and I₃, which are blocked. It is therefore possible to write:

    Q.sub.1 (t.sub.4)+Q.sub.2 (t.sub.4)=Q.sub.1 (t.sub.2)+Q.sub.2 (t.sub.2) (2)

as well as:

    γ.sub.1 (t.sub.4)=γ.sub.2 (t.sub.4)            (3)

because the capacitors C₁ and C₂ then receive the same voltage V_(g) and have substantially the same surface potentials. The distribution of the charges between capacitors C₁ and C₂ at time t₄ is dependent on the values of said capacitors.

At time t₄, there is a quantity of charges beneath capacitor C₂ which is to be transferred during the following stage of the process (stage d) towards the means which convert the charges into an electrical voltage. This same charge quantity will again be present beneath C₂ at time t₂ during the processing of the following sample of the electrical input voltage. It is therefore possible to write:

    C.sub.2 γ.sub.2 (t.sub.2)=C.sub.2 γ.sub.2 (t.sub.4 -T) (4)

Equations (2), (3) and (4) make it possible to write:

    γ.sub.2 (t.sub.4)(C.sub.1 +C.sub.2)=C.sub.1 γ.sub.1 (t.sub.2)+C.sub.2 γ.sub.2 (t.sub.4 -T)

On passing into the frequency range, we obtain: ##EQU4##

FIG. 2f illustrates the stage of the process in which the charges are transferred from the final capacitor C₂ to the means converting the charges into an electrical voltage and more particularly beneath the reading capacitors C_(l). Switch I₃ is conductive and switch I₂ blocked.

The filter is in the state shown in FIG. 2f at time t₅, when only potential φ_(L) is of the high level.

During stage (d) the potential of point P is maintained at a constant value by circuit R, due to the sampling of charges on a capacitor C_(A).

The output signal variation at point S is: ##EQU5## on ignoring the space charge variation.

On passing into the frequency range and taking account of equations (1) and (5), we obtain: ##EQU6##

The transfer function of the filter of FIG. 2a is written: ##EQU7## which is the transfer function of the filter of FIG. 1, the gain g of the amplifier being equal to ##EQU8## The cut-off frequency of the filter according to the invention is regulated by acting on the value of β and therefore on the values of capacitors C₁ and C₂, example by modifying the dimensions L₁ and L₂ of said capacitors. The order of stages (a) and (b) in the process is unimportant, but these stages must be carried out before stage (c), which in turn must precede stage (d).

FIG. 4 illustrates the diagram of a switched capacitor filter of the second order according to the prior art. It is a Sallen Key cell having two series resistors constituted by switches I₁ and I₅ and capacitor C₃ for the first resistor and MOS switches I₆ and I₇ and capacitor C₅ for the second resistor. A capacitor C₆ is connected between one of the terminals of the second resistor and earth, said terminal being connected to the input of an amplifier of gain g, whose output is returned by means of a capacitor C₄ to the common point of the two resistors.

FIG. 5 shows an embodiment according to the invention of the filter of FIG. 4. A network of capacitors and switches of the filter of FIG. 4 is shown in cross-sectional form, parallel to the charge transfer direction indicated by an arrow. FIG. 5 also shows the circuit diagram for this system.

The filter shown in FIG. 5 differs from that of FIG. 2a in that instead of two it has four storage capacitors C₃, C₄, C₅, C₆, separated by MOS switches I₅, I₆, I₇.

The foils of capacitors C₅ and C₆ are connected to V_(g), so that capacitor C₄ used for the looping of the filter during stage (e) of the process receives a voltage equal to V_(g) +ΔV_(S) in which ΔV_(S) is the variation in the output voltage of the filter for the preceding sample of the input signal.

During stage (e) a common surface potential is established between capacitor C₄ and the following capacitor C₅, which is raised to potential V_(g). Capacitor C₄ receives the potential V_(g) +ΔV_(S) from a point B located within the circuit R belonging to the means which convert the charges into electrical voltages and whose circuit diagram will be described in greater detail hereinafter. This circuit R differs from that used for the filter of FIG. 2, which is not looped.

The order of stages (a), (b) and (e) is unimportant, but stages (a), (b) and (e) must be carried out before stage (c), which must precede stage (d).

Circuit R has an MOS transistor T₄ in series with one of the terminals of a capacitor C_(A). A control circuit M₁ is connected to the connecting point A of transistor T₄ and capacitor C_(A). The other terminal of capacitor C_(A) is connected to a point B, whilst an MOS transistor T₆ is connected between point B and a fixed potential V_(p). Point B is connected to capacitor C₄, which ensures the looping of the filter by means of a following stage or an amplifier of unity gain A₁. Between point B and a point S, it is possible to connect a transistor T₉ controlled by φ_(A) and an output stage 20, which ensures the locking and the maintenance of the output voltage at S.

Control circuit M₁ has a transistor T₅ between point A and potential V_(g) and a transistor T₇ between point A and a point g. The grid of T₇, a capacitor C_(g) and a transistor T₈, whose two other terminals are raised to V_(g) are also connected at point g.

FIG. 6 shows diagrams of the signals liable to be applied to the filter of FIG. 5. FIGS. 6a, b, c, d and e show potentials φ_(A), φ_(B), φ₁, φ₂, φ₃ applied to switches I₁ and I₆ and to transistors T₁, T₅ and T₉ in the case of φ_(A), to switches I₅, I₇, transistor T₂ and capacitor C_(g) in the case of φ_(B), to switch I₃ in the case of φ₁, to transistors T₄ and T₆ in the case of φ₂ and finally to transistor T₃ in the case of φ₃.

Like the potentials shown in FIG. 3, potentials φ_(A), φ_(B), φ₁, φ₂, φ₃ are periodic functions of the same cycle T, which is of the square-wave type and whose amplitude can vary between a low level and a high level. Potentials φ_(A) and φ_(B) are preferably identical, but phase-displaced so that they are not simultaneously conductive. Potential φ₁ passes to a high level, when φ_(B) is at the low level and in the embodiment of FIG. 6 φ₁ is at the high level during the period of time when φ_(A) and φ_(B) are at the low level. Potential φ₂ passes to the high level when φ_(A) is at the low level and passes to the low level when φ₁ is still at the high level. Potential φ₃ passes to the high level when φ₂ is at the low level and φ₁ at the high level and passes to the low level before or at the same time as φ₁.

At time t₁ shown in FIG. 6, φ₂ is at the high level. Transistor T₄ functions as a triode, establishing the same potentials at points A and P. Point B is raised to a potential V_(p) by transistor T₆.

At time t₂, φ_(B) and φ₂ are at the high level. Stage (c) of the process is performed. Transistor T₆ is conductive and the voltage at point B is equal to V_(p). It is necessary to choose V_(p) =V_(g), so that the capacitor C₄ is raised to the potential V_(g) during stage (c). Capacitor C_(g) transmits to point g the voltage swing V.sub.φB of the potential φ_(B). Thus, point g passes from v_(g) -V_(T8) to V_(g) -V_(T8) +V.sub.φB, in which V_(T8) is the threshold voltage of transistor T₈. Transistor T₇ starts to become conductive and charges the capacitors C_(A) and C_(l), provided that C_(g) >>C_(A) +C_(l) until the time when transistor T₄ is saturated, thus fixing the potential of point P at V.sub.φ2 -V_(T4), in which V.sub.φ2 corresponds to the high level of potential φ₂ and in which V_(T4) is the threshold voltage of T₄, which is negative because T₄ is an MOS depletion transistor, whilst the other transistors of the circuit are enhancement transistors, whose threshold voltages V_(T) are positive. Point A is at potential V_(AO) =V_(g) +V.sub.φB -V_(T8) -V_(T7).

At time t₃, φ₁ and φ₂ are at the high level and stage (d) of the process takes place. The quantity of charges present beneath the final capacitor C₆ in the charge transfer direction passes beneath the reading capacitor C_(l). The potential of point P is kept constant by T₄, due to a supply of charges sampled from capacitor C_(A), the transistor T₇ being blocked from the passage of φ_(B) to the low level.

The potential at A becomes: ##EQU9## as hereinbefore, E (ω) being the signal present at the input of the filter and H (ω) being the transfer function of the filter of the second order obtained by switching capacitors C₃, C₄, C₅, C₆. The form of said transfer function is fixed by the values of the capacitor C₃, C₄, C₅, C₆ and the gain value of loop ##EQU10##

The processing of a further input signal sample then commences.

Time t₄, φ₁ and φ₃ are at high level and stage (b) of the process takes place. The return to the low level of φ₂ provides the blocking of T₆ and T₄, point B assumes a floating state and points A and P are separated. Point P is connected to the reference potential by T₃ and the quantity of charges Q₆ (t₃) is restored by reading capacitor C_(l) beneath the final capacitor C₆.

At time t₆, φ_(A) is at the high level and stages (a) and (e) of the process take place. The potential of point A is raised to V_(g), so that the potential of point B becomes: V_(B) =V_(g) +V_(p) -V_(A) =V_(g) +V_(p) -V_(AO) +ΔV_(S).

It is necessary to choose V_(p) equal to V_(AO) in order to obtain at point B a voltage equal to V_(g) +ΔV_(S) in which ΔV_(S) is the variation of the output signal of the filter for the preceding signal sample. Transistor T₉ is conductive and transmits the voltage V_(g) +ΔV_(S) to capacitor C₄. The gain g of the amplifier of FIG. 4 is equal to ##EQU11## The output signal is also available at point S.

FIG. 7 shows another embodiment of the filter of FIG. 4 according to the present invention and to which can be applied the signals shown in FIG. 6.

This embodiment differs from that of FIG. 5, because the final capacitor C₆ is not permanently connected to potential V_(g). In FIG. 7, capacitor C₆ is connected to a control circuit M₂, permitting the raising thereof to potential V_(DD) during stage (b) to potential V_(g) during stage (c) and to reference potential during stage (d). The circuit M₂ has three transistors connected to the capacitor C₆ at a point V and also connected to voltage V_(g) for transistor T₂₁, to voltage V_(DD) for transistor T₂₂, whose grid is also raised to V_(DD) and to the reference voltage for transistor T₂₃. This embodiment also differs from that of FIG. 5 through the means which convert the charges into an electrical voltage. These means comprise a reading diode integrated into the semiconductor substrate and separated from the final capacitor C₆ by a switch I₃. Diode D_(l) is connected to a point P, to which is connected a transistor T₁₂, which is also connected to voltage V_(g) and a gain stage k, which can be a following stage constituted by a transistor T₁₀, whose grid is connected to point P and connected between voltage V_(DD) and a point Q, and by an MOS depletion transistor T₁₁ between point Q and voltage -V_(g), whereby the grid of T₁₁ is also connected to -V_(g). Between point Q and point B, a transistor T₄₀ is arranged in series with one of the terminals of a capacitor C_(A), whose other terminal is connected to a point B. Between the common point A of T₄₀ and C_(A) is positioned a transistor T₅, which is also connected to Vg. A transistor T₆ is connected between point B and potential V_(p). As from point B, the circuit diagram is identical to that of the embodiment shown in FIG. 5.

Compared with the embodiment of FIG. 5, potential φ_(A) is also applied to transistor T₁₂, potential φ_(B) to transistor T₂₁, potential φ₁ to transistor T₆ and potential φ₃ to transistors T₂₃ and T₄₀.

At time t₂, stage (c) of the process takes place. Transistor T₂₁ is conductive and raises the final capacitor T₆ to potential V_(g).

At time t₃, the final capacitor C₆ is raised to potential V_(DD) of the order of 12 V by transistor T₂₂. Switch I₃ receives the high level of potential φ₁, which is equal to V_(g) and of the order of 5 V. The low level of φ₁ is taken as being equal to the reference potential.

The level of the potentials beneath C₆ is higher than the level of the potentials beneath switch I₃, as is diagrammatically shown in FIG. 7 which represents the state of the potential walls beneath C₆, I₃ and D_(l) at times t₂, t₃, t₄ and t₅. At time t₃, the transfer of charges from C₆ to D_(l) cannot take place. At time t₃, point B is raised to a potential V_(p) by transistor T₆.

At time t₄, stage (d) of the process takes place. Capacitor C₆ is raised to the reference potential by transistor T₂₃ for which purpose the latter must be much more conductive than T₂₂. There is a transfer of charges from T₆ to beneath diode D_(l), whose potential was restored to V_(g) at time t₆ -T indicated in FIG. 6. Transistor T₄₀ is conductive and capacitor C_(A) is charged. The voltage swing at A is equal to ##EQU12## in which k is the gain of the following stage, Q₆ the quantity of charges present beneath C₆ at time t₃ and C_(D) the capacitance of point P. The following stage can be replaced by an amplifier of gain k equal to 1.

The processing of a further input signal sample starts at time t₅. Stage (b) of the process takes place. Potential φ₃ is at the low level, transistor T₄₀ separates points A and Q and transistor T₂₂ raises the capacitor C₆ to the potential V_(DD). Charges Q₆ are transferred from D_(l) to beneath C₆.

At time t₆, stages (a) and (e) of the process take place. Point A is raised to potential V_(g) by T₅ and the potential of point B becomes: ##EQU13##

It is therefore merely necessary to select potential V_(p) equal to V_(AO) to obtain ##EQU14## in which ΔV_(S) is the variation of the output voltage of the filter for the preceding sample of the electrical input signal. At time t₆, diode D_(l) is restored to potential V_(g) by T₁₂, so that gain g is equal to ##EQU15##

All the transistors used in the invention are preferably MOS enhancement transistors, except for transistors T₁₁ and T₄, which are MOS depletion transistors.

The invention is not limited to the embodiments described and represented hereinbefore and various modifications can be made thereto without passing beyond the scope of the invention. 

What is claimed is:
 1. A switched capacitor filter using a process for filtering an electrical signal by transferring charges into a semiconductor substrate, which is covered with a first insulating layer on which is arranged a first group of electrodes constituting with the first insulating layer and the substrate charge storage capacitors, whilst a second group of electrodes is interposed with the first and is separated therefrom by a second insulating layer, the electrodes of said second group constituting with the first and the second insulating layers and the substrate switches which control the transfer of charges between the storage capacitors, wherein the processing of each sample of the electrical input signal comprises the following stages:(a) a quantity of charges corresponding to the sample of the electrical input signal during processing is introduced beneath the first capacitor (C₃) in the charge transfer direction; (b) beneath the last capacitor (C₆), in the charge transfer direction is introduced a quantity of charges which is equal to that already there, during the processing of the preceding sample of the electrical input signal and prior to the beginning of the transfer of charges from said last capacitor to means which convert the charges into an electrical voltage; (c) a common surface potential is established between two successive capacitors, whereby during the said stage the capacitors are brought to a constant potential V_(g) ; (d) the charges are transferred from the last capacitor in the charge transfer direction to means which convert the charges into an electrical voltage; the order of stages (a) and (b) in the process is random, but stages (a) and (b) must precede stage (c), which must precede stage (d); wherein said filter comprises a diode (D) located in the semiconductor substrate (1) and raised to a constant potential V_(D) and a grid (G_(C)) located in the same plane as the storage capacitors (C₃, C₄, C₅, C₆) and raised to a constant potential V_(DD) which exceeds V_(D), which controls the current supplied by the diode, the grid being separated from the first capacitor in the charge transfer direction by a switch I₁ located in the same plane as the other switches (I₅, I₆, I₇) and the first capacitor being connected to the common point of two transistors T₁ and T₂, arranged in series, and respectively connected to the input signal E and to the voltage V_(g), transistors T₁ and T₂ and switch I₁ being subject to periodic potentials (φ_(A), φ_(B)) such that during stage (a) of the process, transistor T₁ is conductive and switch I₁ is switched off and during stage (c) transistor T₂ conducts.
 2. A switched capacitor filter using a process for filtering an electrical signal by transferring charges into a semiconductor substrate, which is covered with a first insulating layer on which is arranged a first group of electrodes constituting with the first insulating layer and the substrate charge storage capacitors, whilst a second group of electrodes is interposed with the first and is separated therefrom by a second insulating layer the electrodes of said second group constituting with the first and the second insulating layers and the substrate switches which control the transfer of charges between the storage capacitors, wherein the processing of each sample of the electrical input signal comprises the following stages:(a) a quantity of charges corresponding to the sample of the electrical input signal during processing is introduced beneath the first capacitor (C₃) in the charge transfer direction; (b) beneath the last capacitor (C₆), in the charge transfer direction, is introduced a quantity of charges which is equal to that already there, during the processing of the preceding sample of the electrical input signal and prior to the beginning of the transfer of charges from said last capacitor to means which convert the charges into an electrical voltage; (c) a common surface potential is established between two successive capacitors, whereby during the said stage the capacitors are brought to a constant potential V_(g) ; (d) the charges are transferred from the last capacitor in the charge transfer direction to means which convert the charges into an electrical voltage; (e) during the processing of each sample of the electrical input signal, a common surface potential is established between one of the storage capacitors of the charges, ensuring the looping of the filter, and the following capacitor in the charge transfer direction, said capacitors being respectively raised during said stage to voltages V_(g) +ΔV_(S) and V_(g), in which ΔV_(S) is the variation of the output voltage of the filter for the preceding sample of the input signal, the order of the stages (a), (b), and (e) of the process being random, but stages (a), (b), and (e) must precede stage (c) which in turn must precede stage (d); wherein said filter comprises a diode (D) located in the semiconductor substrate (1) and raised to a constant potential V_(D) and a grid (G_(G)) located in the same plane as the storage capacitors (G₃, G₄, G₅, G₆) and raised to a constant potential V_(DD) which exceeds V_(D), which controls the current supplied by the diode, the grid being separated from the first capacitor in the charge transfer direction by a switch I₁ located in the same plane as the other switches (I₅, I₆, I₇) and the first capacitor being connected to the common point of two transistors T₁ and T₂, arranged in series, and respectively connected to the input signal E and to the voltage V_(g), transistors T₁ and T₂ and switch I₁ being subject to periodic potentials (φ_(A), φ_(B)) such that during stage (a) of the process, transistor T₁ is conductive and switch I₁ is switched off and during stage (c) transistor T₂ conducts.
 3. A switched capacitor filter according to claim 1 or to claim 2, wherein the means which convert the charges into an electrical voltage comprise a charge reading capacitor C_(l) located in the same plane as the other storage capacitors and separated from the last storage capacitor (C₆) in the charge transfer direction, by a switch I₃ located in the same plane as the other switches, the surface potential of the substrate being fixed at zero after the reading capacitor by a switch I₄ located in the same plane as the other switches, followed by a zone of the same conductivity as the substrate, but which is strongly doped and which is located beneath the substrate surface, switch I₄ and the highly doped zone being raised to the reference potential of the substrate, wherein the reading capacitor is connected at point P to a circuit R which, during stage (d), maintains point P at a constant potential, by sampling charges from a capacitor C_(A) and supplying the output voltage V_(s) (t) of the filter.
 4. A filter according to claim 3, wherein the circuit R has, connected to point P, a transistor T₄ in series with one of the terminals of capacitor C_(A), a control circuit M₁ being connected to the common point A of the transistor and the capacitor and a transistor T₆ being connected between the other terminal of the capacitor, point B and a fixed potential V_(p) and wherein the transistors T₄ and T₆ and the control circuit M₁ are subject to periodic potentials (φ₂, φ_(A), φ_(B)) such that successively;point B is raised to the potential V_(p) by transistor T₆ control circuit M₁ ensures the charging of capacitor C_(A) and the reading capacitor C_(l) across transistor T₄ and then the saturation of transistor T₄ during stage (d) the saturated transistor T₄ maintains a constant potential at point P by sampling charges from capacitor C_(A) and the potential of point A initially at V_(AO) decreases; the blocking of transistors T₆ and T₄ leads to a floating potential at point B and separates points A and P; during stage (e) of the process point A is raised to potential V_(g) by control circuit M₁ and the potential at point B, with potentials V_(p) and V_(AO) being equal, is V_(g) +ΔV_(S) in which ΔV_(S) is the variation of the output voltage of the filter for the preceding sample of the electrical input signal, said potential V_(g) +ΔV_(S) being applied to the capacitor (C₄), which ensures the looping of the filter.
 5. A filter according to claim 4, wherein the control circuit M₁ has a transistor T₅ between point A and the fixed potential V_(g) and a transistor T₇ between point A and a point g, the grid of T₇ being connected to point g, in the same way as a capacitor C_(g) and a transistor T₈, these two other terminals are raised to potential V_(g), the grid of T₅ and capacitor C_(g) periodically receiving the given potentials (φ_(A), φ_(B)).
 6. A filter according to claim 4, wherein the transistor T₄ is of the MOS depletion type.
 7. A filter according to claim 3, wherein a transistor T₃ is connected between point P and the reference voltage, said transistor T₃ periodically receiving at its grid a given potential φ₃, which brings about the performance of stage (b).
 8. A filter according to claim 1 or claim 2, wherein the last storage capacitor, in the charge transfer direction C₆ is connected to a control circuit M₂ and wherein the means which convert the charges into voltages comprise a reading diode D_(l) integrated into the semiconductor substrate and separated from the last capacitor by a switch I₃, located in the same plane as the other switches, the diode D_(l) being connected to a point P where a transistor T₁₂ is connected between point P and potential V_(g), and a gain stage k is connected between point P and point Q, a transistor T₄₀ being arranged in series with one of the terminals of a capacitor C_(A) between point Q and point B, a transistor T₅ between the common point A of T₄₀ and C_(A) and potential V_(g) and a transistor T₆ being connected between point B and potential V_(p), control circuit M₂, switch I₃ and transistors T₄₀, T₅ and T₆ being subject to periodic potentials (φ_(A), φ_(B), φ₁, φ₃) such that successively:the control circuit M₂ raises the final capacitor to potential V_(g) and stage (c) takes place the control circuit M₂ raises the final capacitor to a potential V_(DD), switch I₃ to potential V_(g) and point B to potential V_(p) by transistor T₆ ; the control circuit M₂ raises the final capacitor to the reference potential and stage (d) takes place, whilst the potential at point A which is initially at V_(AO) developes the blocking of transistor T₄₀ separates points A and Q, the control circuit M₂ raises the final capacitor to potential V_(DD) and stage (b) takes place; stages (a) and (e) take place, point A is raised to potential V_(g) by transistor T₅, the potential of point B with potentials V_(p) and V_(AO) being equal, is equal to V_(g) +ΔV_(S) in which ΔV_(S) is the variation of the output voltage of the filter for the preceding sample of the input signal, said potential V_(g) +ΔV_(S) being applied to the capacitor C₄, which ensures the looping of the filter, and finally D_(l) is restored to potential V_(g) by transistor T₁₂.
 9. A filter according to claim 8, wherein the control circuit M₂ has three transistors connected on the one hand to the final capacitor C₆ at the point V and on the other to the potential V_(g) for transistor T₂₁, to potential V_(DD) for transistor T₂₂, whose grid is connected to V_(DD) and to reference potential for transistor T₂₃, transistors T₂₁ and T₂₃ periodically receiving the given potentials (φ_(B) and φ₃) at their grids.
 10. A filter according to claim 1 or to claim 2, wherein the transistors are of the MOS type. 